ath9k: Filter out beacons from other BSS in STA mode

Passing beacons received from other BSS to s/w in non-scanning
state is unnecessary in STA mode. This patch filters them out in
h/w.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>